1. Technical Field
The present invention relates to a fuel supply system for a diesel engine provided with a pressure fuel accumulator and, more specifically, to a fuel supply system capable of controlling the discharge of a low-pressure fuel pump for supplying fuel to a high-pressure fuel pump.
2. Description of the Related Art
A diesel engine provided with a pressure fuel accumulator for accumulating fuel discharged from a high-pressure fuel pump under pressure, such as disclosed in JP-A 2003-176761, is well known. Generally, in a diesel engine provided with a pressure fuel accumulator, injection quantity of the fuel to be injected in each injection cycle of a fuel injection valve is controlled through the control of injection valve opening duration. Therefore a low pressure fuel pump for supplying the fuel to a high-pressure fuel pump is operated at a fixed discharge rate to adjust the pressure of the fuel in the pressure fuel accumulator to a predetermined pressure. There has been a trend to increase the discharge pressure of the high-pressure fuel pump to improve flammability, and the quantity of the fuel supplied by the low-pressure fuel pump for cooling and lubricating the high-pressure pump has been increased accordingly.
If the low-pressure fuel pump supplies the fuel at a fixed fuel supply rate to the high-pressure fuel pump, the fuel is supplied excessively to the high-pressure fuel pump during idling operation in which the injection quantity is small and, consequently, the quantity of the fuel returned from the high-pressure fuel pump increases. If a return fuel pipe connected to the high-pressure fuel pump is used also as a return fuel pipe connected to the fuel injection valves and the pressure fuel accumulator, a large quantity of the fuel that has returned from the high-pressure fuel pump, in some cases, obstructs the smooth flow of the fuel returned from the fuel injection valves and the pressure fuel accumulator and causes troubles including the deterioration of the accuracy of controlling the injection quantity of the fuel. The weight of a fuel line system increases and the piping of the fuel line system is complicated when a fuel return pipe of a large diameter is used and fuel return pipes are connected individually to the fuel injection valves and the pressure fuel accumulator to return the fuel smoothly.
While the engine is in warm-up operation, in which injection quantity is comparatively small and the quantity of the returned fuel is comparatively large, and the temperature of the fuel is low, wax contained in light oil deposits. Consequently, it is possible that the fuel filter is clogged up with the wax and the fuel cannot be supplied at a predetermined rate when a large quantity of the fuel flows through the fuel filter. If a large quantity of the fuel heated at a high temperature is returned into the fuel tank after the high-load operation of the engine, component parts of the fuel tank including rubber seals are exposed to heat, which affects the life of those component parts adversely. The fuel return pipe of an automotive engine mounted on a vehicle is cooled by running wind while the vehicle is running. Therefore, heating of the return fuel at high temperatures can be suppressed. However, the return fuel is not cooled by running wind while the vehicle is stopping and the engine is in an idling state, the effect of the high-temperature return fuel on the component parts of the fuel tank is significant.